S1

Supporting Information

Thermally Stable Perovskite Solar Cells with Efficiency over 21% via Bifunctional

Additive

Figure S1. Top-view SEM images of perovskite films with biuret additive.

Figure S2. Average grain size obtained from corresponding SEM images in Figure S1 using

Nano Measurer (version 1.2) software.

Electronic Supplementary Material (ESI) for Journal of Materials Chemistry A.This journal is © The Royal Society of Chemistry 2020

S2

Figure S3. XRD pattern of (MAI)·PbI2·DMSO·biuret adduct.

Figure S4. Photographs of perovskite films during crystallization at 100 °C.

S3

Figure S5. Enlarged fingerprint region in the ATR-FTIR spectra for the N-H stretch.

Figure S6. UV-vis absorption spectra of perovskite films with and without biuret additive.

S4

Figure S7. Photovoltaic parameters of MAPbI3 solar cells (12 devices for each case) as a

function of the amount of biuret additive.

Figure S8. IPCE curves and integrated Jsc of the champion devices.

S5

Figure S9. The calculated electron lifetime of control and biuret-modified devices.

S6

Table S1. Photovoltaic parameters of MAPbI3 solar cells with different amounts of biuret

additive. Parameters were averaged over 12 individual devices.

DevicesJsc

(mA cm-2)

Voc

(V)

FF

(%)

PCE

(%)

0 mol% 22.67 ± 0.09 1.06 ± 0.01 75.33 ± 0.42 18.06 ± 0.17

1 mol% 23.29 ± 0.10 1.09 ± 0.01 77.59 ± 0.29 19.74 ± 0.22

2 mol% 23.50 ± 0.08 1.11 ± 0.01 79.42 ± 0.37 20.70 ± 0.75

4 mol% 23.31 ± 0.08 1.10 ± 0.01 78.92 ± 0.47 20.17 ± 0.14

Table S2. Photovoltaic parameters of MAPbI3 solar cells with and without biuret additive

measured under different scan directions.

DevicesScan

direction

Jsc

(mA cm-2)

Voc

(V)

FF

(%)

PCE

(%)HI

Control Reverse 22.38 1.06 76.32 18.15 0.086

Forward 22.41 1.03 71.94 16.59

Biuret Reverse 23.47 1.11 79.65 20.84 0.004

Forward 23.64 1.11 79.17 20.76

S7

Table S3. Summary of the reported device performance of MAPbI3 solar cells.

Device

structure

Advanced

strategiesPCE Year Ref.

ITO/PEDOT:PSS/MAPbI3/C60/BCP

/Ag

Post

treatment21.06%

ACS Nano

2018

Chiang

et al1

ITO/PTAA/MAPbI3/C60/BCP/CuAdditive

engineering21.5%

Joule

2019

Zheng

et al2

ITO/PTAA/Single-Crystal

MAPbI3/C60/BCP/Cu21.09%

ACS Energy

Lett. 2019

Chen

et al3

FTO/cp-TiO2/MAPbI3/Spiro-

OMeTAD/MoO3/Ag

Additive

engineering20.4%

Adv. Mater.

2019

Li et

al4

FTO/TiO2 nanowire/MAPbI3/Spiro-

OMeTAD/Au

Contact

engineering21.10%

Small

2019

Wu et

al5

FTO/cp-TiO2/mp-

TiO2/MAPbI3/Spiro-OMeTAD/Au

Interface

engineering20.4%

Adv. Mater.

2019

Chen

et al6

FTO/cp-SnO2/MAPbI3/Spiro-

OMeTAD/Ag

Contact

engineering20.52%

Adv. Funct.

Mater. 2019

Chen

et al7

FTO/cp-SnO2/mp-

SnO2/MAPbI3/spiro/Au

Contact

engineering19.12%

Adv. Funct.

Mater. 2018

Xiong

et al8

FTO/cp-TiO2/mp-

TiO2/MAPbI3/Spiro-OMeTAD/Au

Additive

engineering21.16% This work

Table S3 summarizes the photovoltaic efficiency of the reported high-performance MAPbI3

solar cells. The reported champion PCE of MAPbI3 solar cells with different structures were

displayed. A promissing PCE of 21.16% is reported in this work which to our knowledge is the

highest efficiency for MAPbI3 solar cells with a mesoporous electron transport layer. Moreover,

the obtained PCE here is even comparable with the PCE of the single-crystal MAPbI3 solar

cells. (note that cp means compact, mp means mesoporous)

S8

Note 1: Crystallite Size Caculation Based on Scherrer Equation

𝐷=𝐾𝜆

𝐵cos 𝜃Here D represents the average crystallite grain diameter (nm), K is the proportionality constant,

λ is the wavelength of the X-ray irradiation (0.154 nm), and B is the full width at half maximum

(FWHM) (in radians). We calculated the crystallite size using the FWHM of the (110) peak.

We assume a proportionality constant of K = 0.94, which is appropriate if the crystallites are

roughly spherical in shape.

𝐷=𝐾𝜆

𝐵cos 𝜃=

0.94 × 0.154

𝐵 ×𝜋180

× cos (14.1°)=8.555𝐵

From an analysis using the Scherrer equation, the crystal sizes of control and biuret-modified

perovkites are estimated to be 51.8 nm and 66.3 nm, respectively. It is important to note that

these values are based on the assumption of spherical perovskite crystals. In contrast, for our

samples, because of the film thickness limitation, crystals are much more parallel than

perpendicular to the substrate, meaning that the crystal size is underestimated by the Scherrer

equation analysis. Considering that all samples have similar film thickness, it is safe to assume

that the observed size trend is still valid.

Experimental Section

Materials

FTO glass (15 Ω/sq) was purchaesd from South China Science & Technology Company

Limited. Titanium diisopropoxide bis(acetylacetonate) was obtained from Sigma-Aldrich.

Methylammonium iodide (MAI, >98.0%(N)), lead (II) iodide (PbI2, 99.99%), and biuret

(>99.0%(N)) were purchased from TCI Chemicals. 2,2′,7,7′-Tetrakis-(N,N-di-

pmethoxyphenylamine)9,9′-spirobifluorene (Spiro-OMeTAD), 4-tert-butylpyridine (tBP),

lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI), and tris(2-(1H-pyrazol-1-yl)-4-tert-

butylpyridine)-cobalt(III) tris(bis(trifluoromethylsulfonyl)imide) (FK209) were purchased

from Xi’an Polymer Light Technology Corp. [6,6]-Phenyl-C61-butyric acid methyl ester

(PCBM, >99.5%) was purchased from Lumtec. Anhydrous solvents, such as

S9

dimethylformamide (DMF), dimethyl sulfoxide (DMSO), chlorobenzene (CB), isopropanol

(IPA), and acetonitrile (ACN), were obtained from Alfa Aesar. All the chemicals were used as

received without further purification.

Device fabrication

The TiO2 compact layer was prepared by spraying a solution containing 1 mL of titanium

diisopropoxide bis(acetylacetonate) and 7 mL of isopropanol on cleaned and patterned FTO

substrates at 460°C using dry air as the carrier gas. Subsequently, the mesoporous TiO2 layer

was spin-coated onto the TiO2 compact layer using diluted pastes and calcined at 510oC for 30

min in air to remove organic components. TiO2 paste was prepared according to previously

reported procedures.9 The MAPbI3 precursor solution was prepared by dissolving 461 mg of

PbI2 and 159 mg of MAI in 700 μL of DMF and 70 μL of DMSO, which was then spin-coated

on the TiO2 mesoporous layer at 4000 rpm (acceleration of 2000 rpm/s) for 30 s, to which 150

μL of CB was poured onto the spinning substrate 20 s prior the end of the program. For the

device with biuret, different amount of biuret was added to the precursor solution. The

perovskite films were then annealed on a hotplate at 100°C for 15 min. Once cooled down to

room temperature, the hole transport layer was deposited on top of the perovskite layer by spin

coating the Spiro-OMeTAD solution at 4000 rpm for 30 s. The Spiro-OMeTAD solution was

prepared by dissolving 73.53 mg (60 mM) of Spiro-OMeTAD in 1 mL chlorobenzene, with the

addition of 29.30 μL (200 mM) of tBP and 17.23 μL (30 mM) of Li-TFSI solution (500 mg Li-

TFSI in 1 mL acetonitrile). Then, 6.78 μL (1.8 mM) of FK209 solution (400 mg FK209 in 1

mL acetonitrile) was added to the Spiro-OMeTAD solution; the molar ratio for FK209 and

Spiro-OMeTAD was 0.03. Finally, a 100 nm gold layer was thermally evaporated on top of the

device.

Characterization

The morphology and crystal structure of the perovskite films were characterized by SEM

(SU8010, Hitachi) and XRD (Smartlab SE, Rigaku), respectively. ATR-FTIR measurements

were conducted with the FTIR spectroscope (IRTracer-100, Shimadzu). XPS were carried out

on the multifunctional photoelectron spectrometer (ESCALAB 250Xi, Thermo Scientific). The

S10

absorption spectra of the perovskite films were measured on a UV-Vis spectrometer (UV-

3600Plus, Shimadzu). Steady-state PL and PL mapping was performed on a home-built system

as described elsewhere.10 The J-V curves were recorded using a Keithley 2400 source meter

under simulated sunlight from Newport AAA solar simulator (AM 1.5, 100 mW cm-2). The

active area of the device was defined as 0.1225 cm2 with a nonreflective metal mask. IPCE

spectra was measured as a function of wavelength from 300 to 900 nm (Enli Technology) with

dual Xenon/quartz halogen light source.

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